This invention relates to optical projection exposure systems for use in the manufacture of semiconductor devices and, more particularly, to reflective, reducing projection exposure optics having very high resolution under illumination of photons with wavelengths less than 200 nm.
Advanced lithographic technologies capable of producing features of .ltoreq.0.2 .mu.m and with high silicon-wafer throughput are needed to meet the demand for larger, faster, and more complex integrated circuits. The present technology for optical projection lithography cannot obtain such a high resolution over a large image field, e.g., an inch square, with a practical depth of focus, i.e., at least 1 .mu.m. The main limitations have been: 1) lack of a source having wavelengths &lt;100 nm with sufficient average power and 2) lack of a high-resolution, low-distortion optical system operating at these short wavelengths.
The free-electron laser (FEL) is now being developed as a source of short-wavelength photons; see, e.g., U.S. Pat. No. 4,917,447, issued Apr. 17, 1990, to Newnam, and U.S. Pat. application Ser. No. 623,866, filed Dec. 7, 1990, now U.S. Pat. No. 5,144,193, issued Sep. 1, 1992, both incorporated herein by reference. Operating at wavelengths less than 200 nm and preferably at wavelengths less than 20 nm, FEL's driven by rf linear accelerators will fulfill the wavelength and average-power source requirements of projection lithography to enable feature resolution of less than 0.1 .mu.m. As used herein, wavelengths less than 200 nm will be referred to as XUV wavelengths. It will be appreciated that a static or dynamic random access memory (SRAM or DRAM) integrated circuit with 25 mm.times.25 mm dimensions and with 0.1 .mu.m features would have about a 1 GByte memory capacity, the equivalent of a present generation supercomputer. Indeed, such an extension of existing optical lithographic technologies will enable the next generation of electronics to be designed and built.
Existing projection optical systems for XUV cannot produce 0.1 .mu.m features over a field of view large enough for the photolithographic production of practical integrated circuits. Resolution of 0.1 .mu.m and 0.05 .mu.m features has been demonstrated by AT&T Bell Laboratories, T. E. Jewell et al., "20:1 Projection Soft X-ray Lithography Using Tri-level Resist," 1263 SPIE Electron-Beam, X-Ray, and Ion-Beam Technology: Submicrometer Lithographies IX, pp. 90-95 (1990), incorporated herein by reference. The magnet undulator in the National Synchrotron Light Source VUV storage ring followed by a pinhole to attain full spatial coherence was the light source. The projection optics was a 20:1-reduction, Schwarzschild two-mirror, two-reflection system. At an exposure wavelength of 36 nm, 0.2 .mu.m lines and spaces were produced in a trilayer resist. Subsequent exposures at 14 nm produced 0.1 .mu.m and 0.05 .mu.m features with small numerical aperture (NA) values of 0.08 and 0.12, respectively. The image field, however, was limited to 25.times.50 .mu.m.
In one prior art optical design (see Design 11, below), two coaxial spherical mirrors are provided in a partially obscured system to provide a 3.3.times.image reduction with four reflections. The general configuration is similar to applicants' configuration shown in FIG. 1. In one design a NA of 0.150 produces an image field of 5 mm.times.5 mm with a resolution of 0.1 .mu.m at a wavelength of 20 nm. In another design, a NA of 0.125 produces an image field of 10 mm.times.10 mm with a resolution of 0.1 .mu.m when illuminated with an incoherent light source at a wavelength of 13 nm. Both designs use a 40% central obscuration. However, even with these small field sizes, the image distortion is as large as 0.25 .mu.m, whereas the required distortion is .ltoreq.0.01 .mu.m for 0.1 .mu.m resolution over the entire image field. It can be shown that the required resolution can be obtained at these wavelengths over a field of only 1.4 mm.times.1.4 mm. At practical field sizes, e.g., 25.4mm.times.25.4mm, the maximum resolution is 0.15 .mu.m using 10 nm illumination with a distortion of 3.6 .mu.m. This is clearly unacceptable performance for the advanced lithography system under development. Accordingly, it is an object of the present invention to resolve image features to .ltoreq.0.2 .mu.m using illuminated wavelengths less than 100 nm.
It is another object of the present invention to provide the desired resolution with a distortion less than 0.1.times.resolution over image fields greater than 10mm.times.10mm.
Yet another object of the present invention is to provide an optical system compatible with existing scanner and stepping equipment for very large area projection.
Still another object of the present invention is to maintain a telecentricity of less than 5 milliradians (mr) to provide an acceptable depth of focus, e.g., about 1 .mu.m.
It is yet another object of the present invention to provide the desired resolution and substantially distortionless image at practical tolerances for optical systems, i.e., surface roughness, dimensional tolerances, alignment tolerances, etc.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.